Automatic fidelity control circuit



Sept 8, 1936- R. AQ BRADEN N 2,053,762

AUTOMATIC FIDELITY CONTROL CIRCUIT ATTORN EY.

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R. A. BRADEN AUTOMATIC FIDELITY CONTROL CIRCUIT Filed March 14, 1935 6 .Sheets-Sheet 3 RENE A,BRADEN ATTORNEY.

Sept. 8, 1936. R A. BRADEN 2,053,762

AUTOMATIC FIDELITY CONTROL CIRCUIT Filed March 14, 1935 6 Sheets-Sheet 4 RENE A. BRADEN ATTORNEY.

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RENE A. BRADEN ATTORNEY.

Sept. 8, 1936. R, A, BRADEN AUTOMATIC FIDELITY CONTROL CIRCUIT Filed March 14, 1935 6 Sheets-Sheet 6 INVENTOR. RENE A. BRADEN www ATTORNEY.

Patented Sept. 8, 1936 UNITED STATES ATENT CFFICE Rene A. Braden, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 14, 1935, Serial No. 10,981

8 Claims. (Cl. Z50-20) My present invention relates to fidelity control 'arrangements for radio frequency signaling systems, and more especially to automatic fidelity, or selectivity, control circuits for radio receivers.

In my co-pending application, Serial No. 700,247, filed November 29, 1933, there are disclosed, and claimed, various arrangements for 'automatically varying the selectivity of a radio receiver when receiving strong signals. In each of the arrangements of the aforesaid application, a double system of signal amplifiers is utilized, certain of the ampliers being operative, in response to variations in amplitude of the received signal energy, to improve the selectivity of the receiver and impair its fidelity when receiving distant station signals, and other of the amplifiers being operative to decrease the selectivity, and improve the iidelity, of the receiver when receiv- 'ing strong signals, as from local stations.

In the present application there are disclosed improved arrangements for automatically varying selectivity and improving iidelity of a radio eceiver when signals of strong carrier amplitude are received. In each of the arrangements to be disclosed hereinafter, a double system of ampliiiers is employed as in the case of the aforesaid application, but the gain control circuits electrically associated with the amplifiers are so arranged that with increasing signal strength the gain of each amplier system decreases, the gain of one of the amplifiers decreasing more rapidly than that of the other system. One of the ampliiier systems is a sharply tuned system, while the other one is a broadly tuned amplier. As

' a consequence of the operation of the gain control circuits, the gain of the sharply tuned ampliiier is much greater than that of the broadly tuned one when weak signals are received and the receiver output acts mostly through the sharply tuned system; while with strong signals the gain of the sharply tuned amplifier is less than that of the broadly tuned circuit, and the receiver output acts solely through the broad circuit.

Accordingly, it may be stated that it is one of the main objects of my present invention to provide at least two parallel amplifiers in a radio receiver, one of which amplifiers has a sharp selectivity characteristic and the other one having a broad selectivity characteristic, the receiver additionally including control circuits for differentially controlling the transmission efficiency of said amplifiers in such a manner that as the signal amplitude input to the receiver increases the gain of both ampliers decreases, the gain of the sharply selective amplifier decreasing more rapidly than that of the amplifier of broad selectivity.

Another important object of the invention is to provide in a radio receiver an amplifier system' `5` incorporating parallel amplifiers of sharp and broad selectivity, the amplifiers having associated with them gain control circuits which function to regulate the gain of each of the amplifiers in such a manner that as the signal carrier amplitude input increases the gain of the sharp amplifier decreases more rapidly than that of the broad amplifier, the system including additional means for causing the gain of the broad amplifier to decrease when receiving signals below a predetermined signal amplitude input level whereby the selectivity is improved when receiving weak signals by cutting the broad amplifier completely out of service.

Another important object of the invention is to* provide in a radio receiver of the superheterodyne type, a pair of intermediate frequency amplifiers, one of which is sharply tuned and the other of which is broadly tuned, the signal input-output relations of the two parallel amplifiers being such gv"2l-' that as the signal input increases; the output of the sharp amplifier at first remains practically constant and then decreases, while the output 4of the broad amplifier starts at a very small value, and increases as rapidly as the output of the sharp amplifier decreases whereby the total output of the intermediate frequency amplifier system remains substantially constant.

Still other objects of the invention are to improve generally automatic fidelity control cir-5V35 cuits for radio receivers, and to provide such control circuits which are not only reliable in operation, but readily constructed and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best Vbe understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. Y

In the drawings:-

Fig. l is a circuit diagram of a receiving system embodying the invention,

Fig. 2 graphically shows the characteristics of the system in Fig. l,

Fig. 3 shows the resonance curve characteristics 55 secured by the arrangement of the system shown in Fig. 1,

Fig. 4 graphically shows additional characteristics secured by the arrangement of Fig. 6,

Fig. 5 shows a circuit diagram of a modified form of the invention for securing the weak signal characteristic shown in Fig. 4,

Fig. 6 is a circuit diagram of a modification wherein the characteristics of Fig. 4 are secured,

Fig. 7 shows another modification of the invention,

Fig. 8 shows an alternative embodiment of the arrangement depicted in Fig. 7.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1, in purely conventional form, a radio broadcast receiving arrangement embodying one form of the invention.

The receiving arrangement comprises a source `of ysignals of intermediate frequency, it being .pointed out that the receiving arrangement is of the superheterodyne type. This source of intermediate frequency signals may include, as is well known to those skilled in the radio art, a source lof signal energy such as a signal collecting means and a tunable radio frequency amplifier; the output of the latter being coupled to a combined oscillator-first detector circuit of any type. In place of the combined type of local `oscillator-first detector circuit there may be utilized a first detector tube upon which is impressed local oscillation energy from any well known vtype of local oscillator. Regardless of the type of receiving arrangement employed prior to the intermediate frequency amplifier network, the

energy of intermediate frequency is impressed upon the input terminals of the pair of parallel I. F. amplifiers shown in Fig. 1.

The -output .of each I. F. amplifier network is impressed upon a second detector of the biased type, and the sum of the output voltages of the two second detectors is impressed upon an audio frequency amplifier network, which may be followed by a suitable reproducer; the network following the second detectors being omitted to preserve simplicity of disclosure. It will, therefore, be noted that between the source of intermediate frequency signals and the input circuit of second detector l, there is disposed an amplifier system which is sharply tuned to the operating inter- A,mediate frequency. Between the intermediate frequency signal source and the input circuit of Ythe second detector 2, there is arranged an amplifier network which is broadly tuned to the .operating intermediate frequency.

The I. F. amplifier system having a sharply selective .characteristic may comprise at least two cascaded amplifier tubes of the screen grid type, the transformer M1 having its primary and secondary windings each resonated to the operating intermediate frequency. Similarly, the transformer M2, coupling the output of the second amplifier tube to the input of the second detector tube l, has its windings each resonated to the desired intermediate frequency. A transformer M3 4Vcouples the amplifier tubes of the broadly selective I. F. amplifier system, while the transformer M4 couples the input electrodes of the second detector 2 to the output of the second of the broadly tuned I. F. amplifier tubes. In the case of each of transformers M3 and M4, the windings thereof are each resonated to the operating intermediate frequency.

In order to secure the different selectivity characteristics for the parallel I. F. amplifier networks, any devices well known to those skilled in the art may be utilized. One method of securing such selectivity characteristics, and it is to be understood that this is given merely by way of illustration, is to adjust the couplings between primary and secondary coils of transformers M1 and M2 to be less than critical coupling so that the resonance curve characteristic of these transformers is relatively sharp; while in the case of transformers M3 and M4, the couplings are designed so that their coupling effects are greater than critical coupling with the result that the circuits associated therewith are given a relatively broad resonance curve characteristic. As a matter of fact, the couplings of M3 and M4 are preferably adjusted so that the resonance curve of the broadly tuned amplifier system has a substantially fiat top. Damping resistors are included in the tuned circuits of the broad amplifier network to insure the latter characteristic.

In order to secure the selectivity control action which is the object of the present invention, there is provided in the plate circuit of the second detector tube l, a series path which includes the choke 3, the detector plate direct current source 4 and resistor 5. A direct current connection 6 is connected between the signal input grid circuit of amplifier l of the sharply tuned I. F. amplifier network and a desired point on the resistor 5 which is at a negative direct current potential with respect to the grounded side of the resistor. This connection to the resistor is preferably adjustable, and it will be understood that the circuit details of the amplifiers preceding detector tube l are not shown because they are well known to those skilled in the art of radio receiving arrangements. It will be understood, of course, that the cathode of amplifier 'I is at a positive potential with respect to the signal input grid thereof, and the cathode circuit of amplifier 7 is completed to ground so that a return path will be provided through the grounded side of resistor 5.

The second detector tube 2 includes in its plate circuit a series arrangement which comprises the choke 3', the detector plate current source 4 and resistor 5'. The direct current connection 6 is connected between the signal input grid circuit of the broadly tuned I. F. amplifier I and a desired point on resistor 5'. The connection to resistor i is made adjustable, and it will be understood that the input circut of the second detector tube 2 includes the tuned secondary circuit of transformer M4 in series with the usual grid biasing source. For a purpose to be described hereinafter in detail, an electron discharge tube 8 has its input electrodes connected across the tuned input circuit of the detector tube 2, the anode of tube 8 being connected by lead 9 to the current source side of choke coil 3.

In Qrder to secure the automatic selectivity control action which is one of the objects of the present invention, there is applied to the broad I. F. amplifier less gain control bias than is applied to the sharply tuned amplifier network. It will -be noted that amplifiers 'l and 1' have their respective gains automatically regulated by the direct current component of the demodulator plate current associated therewith. In other words each of the parallel amplier systems has operatively associated with it an automatic gain control circuit for regulating its gain in response to signal carrier amplitude variations.

The broadly tuned I. F. amplifier network inherently possesses less gain lthan the sharply tuned circuit with the result that with Weak signals it produces practically no output. `When the received signal amplitude is sufficiently high to give appreciable output from the broadly tuned amplifier, the tube 8 begins 4to draw current through resistor 5 which furnishes the automatic gain control bias tothe grids of the sharply tuned amplifier network. At this point it is made clear that the automatic gain control connections may be made from resistors 5 and 5 to as many as desired of the different grid circuits yof the amplifiers of their respective I. F. amplifier systems. The tube 8 Vis-biased sufficiently negative so as to carry practically nocurrent in the absence of a received signal. When a Suniciently strong signal is impressed, the extra current drawn by tub-e 8 increases the negative control bias on the grids of the sharply tuned amplifier, and reduces the gain thereof. In turn this reduces the voltage drop across resistor 5, and allows the gain to increase somewhat; but actually a new equilibrium condition is'reached at which the gain of the sharply tuned amplifier is less than it would have been without the operation of tube 8. In this way increasing received signal amplitude causes the output of the sharply tuned amplifier circuit to decrease when the received signal amplitude reaches the level at which the broad circuits begin to function.

In Fig. 2 there are graphically shown on la logarithmic scale, typical gain-controll characteristic curves for each of the two parallel signal amplifying networks. In deriving these relationships it was first assumed, as an ideal characteristic, that thev amplifiers would be so controlled that the sum of the two signal voltages impressed on the two second detectors would be constantly l volt, thus determining the sum of the amplifications of the two amplifiers, which is designated Total Gain in Fig. 2.V vIt wasthen decided that the gains of the amplifiers should be equal at approximately 3000 or 4000' microvolts input, and in the ratio of 10 to l atlOO microvolts and at 100,000 microvolts input, and vcurves were so drawn. It was assumed that 100 microvolts represented a signal level at which practically the maximum selectivity would beY required` on account of interference, amplifier noise due to high gain, etc.; that 100,000 microvolts was a signal level which would permit operation with the maximum fidelity, or broadness and flatness of -the selectivity curve; and that in the range between 1000 microvolts and 10,000 microvolts, an intermediate degree of fidelity and of selectivity would be most satisfactory.

Smooth curves weredrawn through the three points designated above, and extended so that one (representing the selective amplifier) became asymptotic to the total gain curve at low signal levels, while the other (representing the broad amplifier) becomes asymptotic at high signal level. The broad-circuit curve was finally rounded off to a maximum amplification of,1000 at low input signal amplitude, and corrections were made in the sharp-amplifier curve to keep the total gain constant, the final curves shown in Fig. 2 fulfill the requirements that the total gain must be constant; that at low input signal level the signal must pass principally through the sharp circuit; that at high input level the signal must be amplified principally in the broad circuit; and that there must be a gradual shift from one extreme to the other asV the signal levels raised. These effects are gotten, as is shown by Fig. 2,

by constructing the sharp circuit to have inherently much greater gain than the broad circuit has and causing the gain of each circuit to decreasewith increasing signal strength; that of the sharp circuit decreasing more Arapidly than that of the'broad circuit.

As far as the signal input-signal output relations for the two amplifiers are concerned, such curves would show that as the signal input increases, the output'of the sharply tuned amplifier at first remains practically constant and then decreases, 'while the output of the broadly tuned am-plier starts at a very small value andincreases as rapidly as the output of the sharp amplifier decreases. Y mains substantially constant. It will now be seen why there is utilized in the arrangement of Fig. 1 a device for reducing the output of the sharply tuned amplifier with increasing signal input. However, the arrangement of Fig. 1 is not the only way in which this characteristic canbe obtained, and further modifications of this method are to be hereinafter described. In the arrangements shown in Fig. 1 the broadly tuned amplifier circuits inject an Vauxiliary automatic gain control bias into the sharply tuned amplifier circuit. The adjustment of the gain control connection 6 on resistor 4" is made such that less gain control bias is applied to the broad amplifier than is applied to the sharp circuit, to secure the desired relative ratio of variation of gain with signal strength in the two amplifiers. An additional means for causing the gain of the sharp amplifier to vary more rapidly with input than that of the broad amplifier isA to apply the gain control bias to a greater number of tubes in the sharp amplifier than in the broad amplifier.

In Fig. 3 there is graphically portrayed a group of resonance curves of a system of the type shown in Fig. l, which incorporates parallel sharp and broad amplifiers. It will be observed that these curves are obtained by p-lotting kilocycle's off-resonance against relative output with-constant input. In producing these curves the yresonance curve of the broad amplifier alone was assumed to have a perfectly iiat top and perfectly straight, steep sides, the top of the curve being 20 k. c. wide. The assumed resonance curve of the sharp amplifier is one having the shape of an inverted V. It will be understood that the graphical relationships depicted in Fig. 3, are, therefore, theoretical infnature, and represent the ideal operation sought for by the present invention.V It will be noted that lthe shoulders of each of the resonance curves depend upon the ratio of the gain in the sharp amplifier circuit to the gain in the broad amplifier circuit. If the intermediate frequency signals are actually combined prior to detection, the result would probably be that the shoulders of the resonance curves would be lower due to the different phase angles of the side frequencies of the two signals. However, if the two signals are separately demodulated and the audio currents then added, the final result would be theY same as though the two I. F. signals were added with exact phase correspondence and the combined signals then demodulated.

`By correlating Fig. 3 withV Fig. 2 the signal input voltages corresponding to the various resonance curves in Fig. 3 may be determined. Thus, the curve marked l corresponds to an input of about `3000 microvolts, that marked 100 to' 10 microvolts input, etc. Instead of the gain regu- Hence the total output re# f fao lation curves characteristics shown in Fig. 2, the characteristic curves depicted in Fig. 4 may be secured. These curves show the gain of the broad amplifier decreasing below a received signal amplitude of 100 microvolts. This condition is preferable because it improves the selectivity when receiving Weak signals by cutting the broad amplifier completely out of service. With respect to the signal input-signal output relations ofthe sharp and broad amplifiers for the characteristic shown in Fig. 4, it is pointed out that the output from the broad amplifier will drop more rapidly with decreasing signal input than in the case where the characteristics of Fig. 2y are employed.

The circuit arrangements shown in Fig. 5 can be employed for securing the characteristics shown in Fig. 4. In this case the amplifier 'l has its input grid circuit connected to a desired point on resistor 5 through the adjustable gain control connection B, while the gain control connection 6' from the resistor 5' to the signal input grid circuit of amplifier 'l' includes the negative biasing source l0. It will be observed that tube 8 is dispensed with. A direct current connection is provided between the cathode of tube 1 and thecathode side of resistor R1 which is disposed in the grounded cathode lead of amplifier 1.

As the signal input increases in the circuit of Fig. 5, the gain of the sharply tuned amplifier decreases, and the plate current of the amplifier tube associated with resistor R1 also decreases. This reduces the positive bias on the cathode of tube 1', and permits this tube to pass signals. With further increase of received signal amplitude the automatic gain control circuit of the broad amplifier begins to function, and maintains the output constant. As the received signal amplitude to the sharply tuned amplifier increases, and as soon as the automatic gain control circuit thereof starts to operate, the voltage drop across resistor R1 rapidly falls to a negligible value if the tube 'I is given anv exponential characteristic, and thereafter it has no further effect on the gain of the broad amplifier.

It is not believed necessary to show a particular tube construction for amplifier 'l where the latter is to have an exponential or variable mu, characteristic, since those skilled in the art are fully aware of the construction of such type of tube. Thus it will be seen that the operation of the broadly tuned amplifier is prevented when the received signal amplitude is very weak. 'Ihe negative biasing source l in lead 6 applies a negative bias to thegrid of tube 1 sufficiently large to reduce the gain of amplifier 1' substantially to zero when no signals are received by the system. At the same time tube 1, lacking this bias,` has a high gain as the cathode bias is not enough to seriously reduce the gain of tube 1 with no-signal reception. Of course, a common source of I. F. energy feeds the parallel amplifier networks.

The arrangement shown in Fig. does not include a device for reducing the gain of the sharply tuned amplifier at very high signal input levels, but means for accomplishing this is readily added. One manner of securing this additional effect is disclosed in the circuit arrangement of Fig. 6. In other words both the weak and strong signal Ycharacteristics of Fig. 4 are secured in the circuit arrangement shown in Fig. 6. Considering, then, the circuit details of the system shown in Fig. 6, it will be observed that the first amplier tube 1 of the sharply tuned amplifier system includes vof amplifier 1.

the resistor Rr in its grounded cathode lead, and Ythat the direct current connection I is arranged between theY cathode side of resistor R1 and the cathode of the amplifier 'l' of the broad amplifier network.

The purpose of this connection is the same as Vthat shown in Fig. 5; it functions to reduce the gain of the broad I. F. amplifier substantially to zero when' signals of less than a predetermined amplitude are received. The electron discharge tube 20, however, is added inv order to reduce the gain of the sharp amplifier at very high signal input levels. To accomplish this the plate circuit of tube 20 includes in series with the direct current supply source the resistor 2|, the cathode of tube 20 being grounded, and resistor 2| having one side thereof grounded. The signal input grid of tube 26 is connected to a source of negative bias through resistor 22, and condenser 23 connects the signal input grid of tube 20 to the high alternating potential side of the second amplifier 24 inv the sharply tuned amplifier system. The signal input grid circuits of amplifiers l and 24 are connected by the gain control connections 25 to a desired point of negative potential on resistor 2|. The connection to resistor 2| is made adjustable.

In the case of the broadly tuned amplifier the signal input grid circuitsof amplifiers 'l' and 24 are connected to a desiredr point on resistor 26 by means of the direct current connections 21. 'Ihe connectionto resistor 26 is made adjustable, and it will be notedA that the negative bias source I0 is connected so as to furnish the auxiliary nosignal cut-ofi bias solely on the signal input grid It is to be understood that in the case of the modification shown in Fig. 6, as well as those shown in Figs. 1, 5, 7, and 8, the resonanc-e curve characteristics of the sharp and broad amplifiers may be secured in the same manner as described in connection with Fig. l. Further, it is pointed out that the resonance curve characteristics shown in Fig. 3 are the characteristics sought to be obtained asthe received signal amplitude varies in magnitude from maximum to minimum. It is desired to automatically secure an apparent resonance curve characteristic for the entire system which has a substantially flat top and sharp steep sides when receiving strong signals,` whereas it is desired to secure a sharp resonance characteristic shaped like an inverted V when receiving weak signals.

In the arrangement shown in Fig. 6 the gain control tube 20 for the sharp amplifier circuit is tapped onto the grid of the last I. F. amplifier tube, and controls the tubes ahead of this last tube so asV to get practically constant signal voltage at the grid of the last I. F. amplifier tube. In addition, the last amplifier tube is regulated automatically as to gain, and therefore the signal impressed on the second detector of the sharp amplifi-er system decreases with increasing signal input. The broadly tunedl amplifier circuit is controlled as described in connection with Fig. 5. In connection with the arrangements shown in Figs. 5 and 6, it is pointed out that the gain control action on the broad amplifier is intentionally made less flat than that on the sharply tuned circuit so that the input-output curve characteristic of the broad amplifier will be a steadily increasing one instead` of one wherein there is an increase of output with. input up to a predetermined pcint and then a flattening out of the curve.

In this way the output from the broad amplifier circuit Continues to increase as that of the sharply tuned circuit decreases, and the total output remains substantially constant with increasing input. It will, therefore, be seen that in the arrangement of Fig. 6 the broad amplifier is substantially inoperative for weak signals having an amplitude below a predetermined magnitude; as this amplitude is exceeded the voltage drop across resistor R1 is decreased due to the automatic gain control action on amplifier 1, and thus the positive bias on the cathode of amplifier 'l' is reduced. In this way the broad amplifier is rendered operative. Due to the automatic gain control circuit design associated with the broad amplifier the gain of the broad amplifier will increase up to a predetermined signal amplitude while the gain of the sharp amplier is being decreased, and thereafter the gain of the broad amplifier will be reduced simultaneously with the reduction of the gain of the sharp amplier. Of course, the gain control characteristics correspond to those shown in Fig. 4.

The modification shown in Fig. 7 employs in conjunction with the sharp I. F. amplifiers 1 and 24, the control tube 20. The automatic gain control bias impressed by connection 25 on the signal grid input circuit of the amplifier 1 by Virtue of the drop across resistor 2| maintains a substantially constant signal voltage at the grid of the last amplifier 24. The automatic gain control connection on the broad I. F. amplifier which includes tube 1' and 24', functions solely as described in connection with Figs. 5 and 6. The gain control connection 21 is adjustably tapped to resistor 26. As the signal output from the broad I. F. amplifier circuit increases, the voltage drop across resistor 26 is applied as an auxiliary automatic gain control bias to the signal input grid of the last amplifier 24 in the sharply tuned system, with the result that the signal at the following second detector grid is decreased as further increase of signal strength occurs. To accomplish this the direct current connection 30 is provided between the signal input grid circuit of tube 24 and resistor 26. The connection 30 is made adjustable on resistor 26. Y

Althoughthe diagram (Fig. 7 )Y shows the sharp circuit gain control connected to one tube and the broad circuit gain control connected to two tubes, it is to be understood that if the sharp amplifier comprised several tubes, the control voltage would be applied preferably to all except the last one, whereas the broad circuit control bias might be applied to all the tubes in the broad amplifier, or preferably to some but not all of them, so as to cause the broad circuit gain control to operate at a slower rate with change of input, than the sharp circuit gain control. Supplementing this, the resistance 2| may be considerably larger than 26, so that a larger gain control voltage is developed across 2| than across 26. This arrangement serves to slow down the gain control action in the broad circuit relative to that in the broad circuit.

The selectivity control connection shown in Fig. 8 is similar to that employed in the circuit arrangement of Fig. 7. The auxiliary gain control action on the sharp I. F. amplifier is modified, however, in that the gain control bias is secured by the voltage drop across resistor 2| which is disposed in the space current path of control tube 20. The cathode circuits of the amplifiers 1 and 24 of the broadly tuned amplifier include resistors for providing initial bias on the ampliers 1 and 24. It is not believed necessary to explain the action of the arrangements shown in Fig. 8 since that given in connection with Fig. 7 is equally applicable. It is merely necessary to point out that as the signal input to tube 20 increases, the voltage drop across resistor 2| increases with the result that the gain of amplifier 24 is reduced for very strong signals. The voltage at grid of 24 does vary somewhat with signal input to the amplier, since the AVC action is not perfect. This slight variation is sufiiciently amplified by tube 20 to give enough control on 24. The arrangement in Fig. 8 differs from that shown in Fig. 6 in that tubes and 24 are tapped separately onto resistance 2|, whereas in Fig. 6, they are tapped to the same point. Fig. 6 includes, also, the broadchannel suppressor operating at low input amplitude.

It is to be understood that the circuits shown herein are not restricted to. superheterodyne'sets. The parallel amplifier networks may be of the tuned radio frequency type. Again, any of the methods of separating and recombining the signals shown in the aforesaid co-pending application may be utilized herein. While damping resistors are only shown used in the tuned circuits of the broad amplifier of Fig. 1, the same elements may obviously be used in the broad amplifier net-- works of the remaining figures for the samepurpose. Certain of the amplifier cathode circuits have been conventionally represented; those skilled in the art will appreciate that they are to include the usual grid bias resistors.

While I have indicated and described several systems for carrying my invention into effect; it

voltages in a decreasing sense, and at differentv rates, as the received signal amplitude increases.

2. In `a signal receiving system comprising a pair of sharp and broad amplifiers, the method of controlling the selectivity of said system which includes rectifying the signal output of each ampliiier to provide a control voltage which varies inthe same sense as the signal amplitude, decreasing the gain of each amplifier with its respective control voltage as the signal amplitude increases, derivingv an auxiliary control voltage from the signals, and utilizing the auxiliary voltage to decrease the sharp amplifier gain at a faster rate with signal amplitude increase.

3. In combination, two parallel amplifiers for modulated carrier currents having respectively sharp and broad selectivity characteristicseach amplifier including cascaded tubes, a common connection feeding signals into both amplifiers from the same source, a common output connection, separate automatic gain control means connected to each amplifier, the gain control means for the sharp amplifier being designed to regulate closely so as to hold signal carrier amplitude practically constant at the grid of the last amplifier tube, the gain control means for the broad amplier regulating less closely so that increasing input amplitude causes output amplitude to the last tube, so that the gain decreases as the increase, additional means associated with the sharp amplifier for controlling the gain of the last tube, so that the gain decreases as the signal amplitude impressed on the amplifier Vincreases thus causing the output amplitude to decrease as the input amplitude increases, the rate of decrease of the sharp amplifier output being so proportioned to the rate of increase of the broad amplifier output, with increasing input amplitude, that the total output of the two amplifiers is maintained constant, while the ratio of output from the broad amplifier to that from the sharp amplier increases with increasing signal input amplitude.

c 4. In combination, two parallel amplifiers for modulated carrier currents having respectively sharp and broad selectivity characteristics, each amplifier including cascaded tubes, a common connection feeding signals into both amplifiers from 'the'same source, a common output connection, separate automatic gain control means connected to .each amplifier, the gain control means for the sharp amplifier being designed to regulate closely so as to hold signal cariier amplitude practically constant at the grid of the last amplifier tube, the gain control means for the broad amplifier regulating less closely so that increasing'input amplitude causes output amplitude to increase, additional means associated with the sharp amplifier for controlling the gain of signal amplitude impressed on the amplifier increases thus causing the output amplitude to decrease as the input amplitude increases, the rate of decrease of the sharp amplifier output being so proportioned to the rate of increase of the broad amplifier output, with increasing input amplitude, that the total output of the two amplifiers is maintained constant, while the ratio of output from the broad amplifier to that from the sharp amplifier increases with increasing signal input amplitude, and said additional means being connected to derive its bias voltage from signals impressed on the last sharp amplifier tube.

5. In combination, two parallel ampliers for modulated carrier currents having respectively sharp and broad selectivity characteristics, each amplifier including cascaded tubes, a common connection feeding signals into both amplifiers from the same source, a common output connection, separate automatic gain control means connected to each amplifier, the gain control means for the sharp amplifier being designed to regulate closely so as to hold signal carrier amplitude practically constant at the grid of the last "amplifier tube, the gain control means for the broad amplifier regulating less closely so that increasing input amplitude causes output amplitude to increase, additional means associated with the sharp amplifier for controlling the gain of the last tube, so that the gain decreases as the signal amplitude impressed on the amplifier increases thus causing the output amplitude to decrease Yas the input amplitude increases, the rate of decrease of the sharp amplifier output being so proportioned to the rate of increase of the broad amplifier output, with increasing input amplitude, that the total output of the vtwo amplifiers is maintained constant, while the ratio of output from the broad amplifier to that from 'the sharp amplifier increases with increasing signal input amplitude, and said sharp amplifier gaincontrol means controlling all except the last ofthe sharp amplifier tubes with a bias voltage 7A derivedY from the` signal, amplitude at the grid of the last tube, and said additional means controlling the last tube of the sharp amplifier with a bias voltage derived from the signal output of the broad amplifier in such a way that as the output of the latter increases, the gain of the last tube of the sharp amplifier is caused to decrease.

6. In combination, two parallel amplifiers for modulated carrier currents having respectively sharp and broad selectivity characteristics, each amplifier including cascaded tubes, a common connection feeding signals into both ampliers from the same source, a common output connection, separate automatic gain control means corinected to each amplifier, the gain control means for the sharp amplifier being designed to regulate closely so as to hold signal carrier amplitude practically constant at the grid of the last amplifier tube, the gain control means for the broad amplifier regulating less closely so that increasing input amplitude causes output amplitude to increase, additional means associated with the sharp amplifier for controlling the gain of the last tube, so that the gain decreases as the signal amplitude impressed on the amplifier increases thus causing the output amplitude to decrease as the input amplitude increases, the rate of decrease of the sharp amplifier output being so proportioned to the rate of increase of the broad amplifier output, with increasing input amplitude, that the total output of the two amplifiers is maintained constant, while the ratio of output from the bi'oad amplifier to that from the sharp amplifier increases with increasing input amplitude. means associated with the sharp amplifier for producing a voltage which varies inversely with the signal input amplitude, and connections whereby this voltage is applied as an auxiliary gain-controlling grid bias to one of the tubes in the broad amplifier, whereby the gain of the broad amplifier is reduced practically to zero when the input signal is below a predetermined amplitude, and is controlled by the broad amplifier automatic gain-control circuit when the input is above this amplitude, the auxiliary bias then being reduced substantially to zero.

7. In combination, two parallel amplifiers for modulated carrier currents having respectively sharp and broad selectivity characteristics, each amplifier including cascaded tubes, a common connection feeding signals into both amplifiers from the same source, a common output connection, separate automatic gain control means connected to each amplifier, the gain control means for the sharp amplifier being designed to regulate closely so as to hold signal cariier amplitude practically constant at the grid of the last amplifier tube, the gain control means for the broad amplifier regulating less closely so that increasing input amplitude causes output amplitude to increase, additional means associated with the sharp amplifier for controlling the gain of the last tube, so that the gain decreases as the signal amplitude impressed on the amplifier increases thus causing the output amplitude to decrease as the input amplitude increases, the rate of decrease of the sharp amplifier output being so proportioned to the rate of increase of the broad 8. A high-gain sharp amplifier and detector With automatic gain-control means which cause the output to decrease as the input amplitude increases, a 10W-gain broad amplifier and detector with automatic gain-control means which cause the output to increase as the input amplitude increases, the rates of change of output amplitude with respect to a change of input amplitude being so related that the total output of the two detectors is held substantially constant as the input amplitude Varies, while the ratio of output from the broad amplier to output from the sharp amplifier Varies from nearly zero with small input amplitude to nearly innity with large input amplitude.

RENE A. BRADEN. 

